1. Field of the Invention
The present invention relates to a position transducer used in the fields of machine tools, industrial machinery, etc. to detect a position changed due to a rectilinear movement, rotational movement or the like.
This application claims the priority of the Japanese Patent Application No. 2002-064566 filed on Mar. 8, 2002, the entirety of which is incorporated by reference herein.
2. Description of the Related Art
The position transducers used in th fields of machine tools, industrial machinery, etc. to detect a position changed due to a rectilinear movement or the like are known from the disclosure in the Japanese Unexamined Application Publication Nos. 99564 of 1985, 177019 of 1988, 152314 of 1989, etc.
Referring now to FIG. 1, there is schematically illustrated in the form of a block diagram a conventional position transducer with one absolute track. The position transducer is generally indicated with a reference 120. As shown, the position transducer 120 is composed of one absolute track 121 and an ABS head 124 including n absolute track detectors 1231, 1232, . . . , 123a. It should be noted that in the following explanation, the absolute track will be referred to as xe2x80x9cABS trackxe2x80x9d and the ABS track detector be referred to as xe2x80x9cABS detectorxe2x80x9d.
The ABS track 121 is formed from a pattern in which micro-areas each represented by xe2x80x9c0xe2x80x9d and ones each represented by xe2x80x9c1xe2x80x9d are laid in array according to an a-order cyclic code. The micro-area represented by xe2x80x9c0xe2x80x9d and one represented by xe2x80x9c1xe2x80x9d are different in physical property from each other. For example, for the position transducer 120 to magnetically detect the position of the ABS head 124, the micro-areas xe2x80x9c1xe2x80x9d are magnetized but the micro-areas xe2x80x9c0xe2x80x9d are not.
The ABS head 124 is movable longitudinally of the ABS track 121 as indicated by an arrow Y in FIG. 1. Each of the ABS detectors 1231 to 123a is provided in a position opposite to the ABS track 121. Also, when the micro-areas are laid at a pitch xcex, the ABS detectors 1231 to 123a are spaced a distance xcex corresponding to the micro-area pitch xcex from each other.
In the aforementioned position transducer 120, since the ABS track 121 is formed as the pattern in which the micro-areas each represented by xe2x80x9c0xe2x80x9d and ones each represented by xe2x80x9c1xe2x80x9d are laid in array according to the a-order cyclic code, as the ABS head 124 moves in relation to the ABS track 121, the ABS head 124 will detect ABS values all different from each other irrespectively of the fact that only the ABS track 121 is included in the position transducer 120. Therefore, the position transducer 120 can detect the position of the ABS head 124 in relation to the ABS track 121 on the basis of the ABS values detected by the ABS head 124.
The position transducer 120 can have the ABS track 121 reduced in size in a direction indicated by an arrow Z in FIG. 1 because it has only one ABS track 121. That is, the position transducer 120 can be designed smaller.
In the position transducer 120 having only the ABS track 121, when the position of the ABS head 124 varies, signals outputs from two or more of the ABS detectors 1231 to 123n will possibly be switched from one to another. Specifically, signal outputs from two or more of the ABS detectors 1231 to 123n are switched from xe2x80x9c0xe2x80x9d to xe2x80x9c1xe2x80x9d or vice versa.
It is difficult to provide all the ABS detectors 1231 to 123n having quite the same capability of detecting a physical property and outputting a signal. Also it is difficult to provide all the micro-areas spaced at quite the same pitch xcex (as shown in FIG. 1) from each other when producing the ABS track 121.
Therefore, when each of the ABS detectors 1231 to 123n is opposite to near the edge of a micro-area, it is difficult for the ABS head 124 to accurately detect a code from the ABS track 121 because the ABS detectors 1231 to 123n output signals at different times, respectively.
Also, when each of the ABS detectors 1231 to 123a is opposite to near the edge of a micro-area, the magnetic field to be detected is influenced by the adjacent micro-areas. Therefore, when a magnetized micro-area is adjacent to a micro-area not magnetized, there will take place near the edge of each micro-area an area where it is difficult for the ABS detector to stably output xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d signal.
Namely, when each of the ABS detectors 1231 to 123a is near the edge of a micro-area, it will be difficult for the position transducer 120 to accurately detect the position of the ABS head 124 in relation to the ABS track 121.
Position transducers capable of detecting the position of the ABS head in relation to the ABS track without error include two types which will be described below.
The first one of the two types of position transducers is disclosed in the Japanese Unexampled Application Publication No. 1990-21216, Japanese Patent No. 2679207 and Japanese Unexamined Application Publication No. 1988-152314, respectively. This position transducer does not detect signal output from each of the ABS detectors that is opposite to near the edge of a micro-area but detects signal output from each of the ABS detectors that is opposite to near the center of the micro-area (which area will be referred to as xe2x80x9cstable areaxe2x80x9d hereunder).
FIG. 2 is also a block diagram of a variant of the aforementioned conventional position transducer. This conventional position transducer is generally indicated with a reference 140. As shown, in the position transducer 140, an incremental track 142 having a pattern of constant-wavelength, alternating magnetic fields is juxtaposed with the ABS track 121. Also, the position transducer 140 includes, in addition to the ABS head 124, an incremental track head 144 having two incremental track detectors 1431 and 1432.
The position transducer 140 is adapted such that judging from a signal output from the incremental track head 144 that each of the ABS detectors 1231 to 123a is in the stable area, it detects signal outputs from the ABS detectors 1231 to 123a.
When each of the ABS detectors 1231 to 123a is not in the stable area, however, the aforementioned position transducer 140 cannot detect any position of the ABS head 124 in relation to the ABS track 121.
Therefore, for example, to detect the position of the ABS head 124 in relation to the ABS track 121 the moment the power is applied to the position transducer 140, each of the ABS detectors 1231 to 123a have to be in the stable area at that moment. If none of the ABS detectors 1231 to 123a are in their respective stable areas, the position transducer 140 cannot detect any position of the ABS head 124 in relation to the ABS track 121 before the ABS head 124 is moved until each of the ABS detectors 1231 to 123a moves into the stable area. Namely, the position transducer 140 can hardly detect the position of the ABS bead 124 in relation to the ABS track 121 the movement it is applied with the power.
The second type of position transducer is disclosed in the Japanese Patent Nos. 2571393, 2571394, 3063044 and 3103266. The positional transducer includes a sets of ABS detectors, each set including two ABS detectors. In this position transducer, signal output from one of the two ABS detectors that is opposite to the stable area is used to detect an a-bit code.
In this position transducer, however, the ABS head includes 2n ABS detectors to detect an n-bit code indicating an a-bit code. That is, the position transducer will have a larger number of ABS detectors for the number of bits in a code the ABS head detects. Since many ABS detections are made in the position transducer, the ABS head will possibly have to be designed large and corresponding addition of wires and the like will possibly cause the circuit to be more complicated. Therefore, this position transducer can hardly be designed compact. Also, since the position transducer is produced through many steps, it is difficult to produce it with less costs.
Further, the Japanese Unexamined Application Publication No. 1990-284025 discloses a positional transducer which detects an a-bit code by an ABS head composed of (a+xcex1) (xcex1 is an integer larger than 1) ABS detectors nearly equidistantly spaced from each other.
In the above position transducer, the ABS detectors are so disposed that when the ABS head is moved in relation o the ABS track, one ABS detector at the maximum will be opposite to near the boundary between a micro-area and a one adjacent to the micro-area, and the value xcex1 is decided so that one code will take place in the effective length of measurement taking in account a case in which a code is detected by the ABS head including also an ABS detector whose output is unstable.
In this position transducer, however, the value xcex1 has to be 7 when the value a=8 for example as described in the above Japanese Unexamined Application Publication No. 1990-284025. Namely, the number of ABS detectors provided in the position transducer is 2axe2x88x921. Therefore, for this position transducer, it is difficult to design the ABS head compact and simplify the circuit.
Also, in the above position transducer, if xcex1 less than 7 when a=8, one code will take place twice in the effective length of measurement. Therefore, when xcex1 less than 7, only a part of the ABS track in the position transducer will be usable. Namely, since the effective length of measurement is reduced, the detectable range will be narrower.
It is therefore an object of the present invention to overcome the above-mentioned drawbacks of the related art by providing a position transducer capable of accurately detecting an n-bit code by an absolute track head including n+m (n is a larger integer than 3 and m is a larger integer than 1) absolute track detectors even when it includes one absolute track and also capable of detecting the position of the absolute track head in relation to the absolute track independently of the position of each absolute track detector.
The above object can be attained by providing a position transducer including according to the present invention:
an absolute track in which micro-areas each represented by xe2x80x9c0xe2x80x9d and those each represented by xe2x80x9c1xe2x80x9d, when two types of micro-areas different in signal-selective physical property from each other are represented by either of two codes xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d, are laid in array to provide an n-bit code (n is a larger integer than 3);
an absolute track head having (n+m) (m is a larger integer than 1) absolute track detectors laid in array oppositely to the absolute track and which is movable in relation to the absolute track to detect the physical property of the absolute track;
an intra-micro-area position detecting means for detecting the position of each absolute track detector in the micro-area;
a signal selecting means for selecting n ones of (n+m) signal outputs from the absolute track detectors on the basis of the result of detection by the intra-micro-area position detecting means; and
a code deciding means for the n-bit code on the basis of signal output from an absolute track detector selected by the signal selecting means;
a distance xcex1 between the first one and (n+m)th one of the absolute track detectors satisfying a requirement that (nxe2x88x921)xcex+2xcex4 less than xcex1 (where xcex greater than 0 and xcex4 greater than 0, xcex is the minimum resolution length of signal output from each absolute track detector and xcex4 in an unstable area is a length in the moving direction of the absolute track head); and
a distance xcex2 between the absolute track detectors satisfying a requirement that 2xcex4 less than xcex2 less than xcexxe2x88x922xcex4.
These objects and other objects, features and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments of the present invention when taken in conjunction with the accompanying drawings.